Network Working Group R. Bush, Ed.
Internet-Draft Internet Initiative Japan
Intended status: Experimental January 5, 2011
Expires: July 9, 2011
The A+P Approach to the IPv4 Address Shortagedraft-ymbk-aplusp-07
Abstract
We are facing the exhaustion of the IANA IPv4 free IP address pool.
Unfortunately, IPv6 is not yet deployed widely enough to fully
replace IPv4, and it is unrealistic to expect that this is going to
change before the depletion of IPv4 addresses. Letting hosts
seamlessly communicate in an IPv4-world without assigning a unique
globally routable IPv4 address to each of them is a challenging
problem.
This draft proposes an IPv4 address sharing scheme, treating some of
the port number bits as part of an extended IPv4 address (Address
plus Port, or A+P). Instead of assigning a single IPv4 address to a
single customer device, we propose to extend the address field by
using bits from the port number range in the TCP/UDP header as
additional end point identifiers, thus leaving a reduced range of
ports available to applications. This means assigning the same IPv4
address to multiple clients (e.g., CPE, mobile phones), each with its
assigned port-range. In the face of IPv4 address exhaustion, the
need for addresses is stronger than the need to be able to address
thousands of applications on a single host. If address translation
is needed, the end-user should be in control of the translation
process - not some smart boxes in the core.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. This document may not be modified,
and derivative works of it may not be created, and it may not be
published except as an Internet-Draft.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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working documents as Internet-Drafts. The list of current Internet-
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This Internet-Draft will expire on July 9, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
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Internet-Draft A+P Addressing Extension January 20111. Introduction
This document describes a technique to deal with the imminent IPv4
address space exhaustion. Many large Internet Service Providers
(ISPs) face the problem that their networks' customer edges are so
large that it will soon not be possible to provide each customer with
a unique public IPv4 address. Therefore, although undesirable,
address sharing, in the same molds as NAT, is inevitable.
To allow end-to-end connectivity between IPv4 speaking applications
we propose to extend the semantics of the IPv4 address with bits from
the UDP/TCP header. Assuming we could limit the applications' port
addressing to any number of bits lower than 16, we can increase the
effective size of an IPv4 address by remaining additional bits of up
to 16. In this scenario, 1 to 65536 customers could be multiplexed
on the same IPv4 address, while allowing them a fixed or dynamic
range of 1 to 65536 ports. Customers could for example receive
initial fixed port range, defined by operator and dynamically request
additional blocks, depending on their contract. We call this
"extended addressing" or "A+P" (Address plus Port) addressing. The
main advantage of A+P is that it preserves the Internet "end-to-end"
paradigm by not requiring translation (at least for some ports) of an
IP address.
1.1. Problems with Carrier Grade NATs
Various forms of NATs will be installed at various levels and places
in the IPv4-Internet to achieve address compression. This document
argues for mechanisms where this happens as close to the edge of the
network as possible, thereby minimizing damage to the End-to-End
Principle and allowing end-customers to retain control over the
address and port translation. Therefore it is essential to create
mechanisms to "bypass" NATs in the core when applicable and keep the
control at the end-user.
With Carrier Grade NATs in the core of the network the user is
trapped behind unchangeable application policies, and the deployment
of new applications is hindered by the need to implement the
corresponding Application Level Gateways (ALGs) on the CGNs. This is
the opposite of the "end-to-end" model of the Internet.
With the smarts at the edges, one can easily deploy new applications
between consenting end-points by merely tweaking the NATs at the
corresponding Customer Premises Equipment (CPE), or even upgrading
them to a new version that supports a specific ALG.
Today's NATs are typically mitigated by offering the customers
limited control over them, e.g. port forwarding or UPnP/NAT-PMP.
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However, this is not expected to work with CGNs. CGN proposals -
other than DS-Lite [I-D.ietf-softwire-dual-stack-lite] with A+P or
PCP [I-D.wing-softwire-port-control-protocol]- admit that it is not
expected that applications that require specific port assignment or
port mapping from the NAT box will keep working.
Another issue with CGN is the trade-off between session state and
network placement. The furthest from the edge the CGN placed, the
more session state needs to be kept due to larger subscriber
aggregation, and more disruption in the case of a failure. In order
to reduce the state, CGNs would end up somewhere closer to the edge.
The CGN hence trades scalability for the amount of state that needs
to be kept, which makes optimally placing a CGN a hard engineering
problem
In some deployment scenarios, CGN can be seen as single point of
failure and therefore the availability of delivered services are
impacted by the ones of CGN s devices. Means to ensure state
synchronisation and failover would be required to allow for service
continuity whenever a failure occurs.
Intra-domain paths may not be optimal for communications between two
nodes connected to the same domain deploying CGNs, hence leading to
path stretches.
2. Terminology
This document makes use of the following terms:
Public Realm: This realm contains only public routable IPv4
addresses. Packets in this realm are forwarded based on the
destination IPv4 address
A+P Realm: This realm contains both public routable IPv4 and also
A+P addresses.
A+P Packet: A regular IPv4 packet is forwarded based on the
destination IPv4 address and the TCP/UDP port numbers.
Private Realm: This realm contains IPv4 addresses that are not
globally routed. They may be taken from the [RFC1918] range.
However, this draft does not make such an assumption. We regard
as private address space any IPv4 address, which needs to be
translated in order to gain global connectivity, irrespective of
whether it falls in [RFC1918] space or not.
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Port Range Router (PRR): A device that forwards A+P packets.
Customer Premises Equipment (CPE): cable/DSL modem.
Provider Edge Router (PE): Customer aggregation router
Provider Border Router (BR): Providers edge to other providers
Network Core Routers (Core): Provider routers which are not at the
edges.
3. Design Constraints and Functions
The problem of address space shortage is first felt by providers with
a very large end-user customer base, such as broadband providers and
mobile service providers. Though the cases and requirements are
slightly different, they share many commonalities. In the following
we develop a set of overall design constraints for solutions
addressing the IPv4 address shortage problem.
3.1. Design Constraints
We regard several constraints as important for our design:
1) End-to-End is under customer control: Customers shall have
the ability to deploy new application protocols at will.
IPv4 address shortage should not be a license to break the
Internet's end-to-end paradigm.
2) Backward compatibility: Approaches should be transparent to
unaware users. Devices or existing applications should be
able to work without modification. Emergence of new
applications should not be limited.
3) Highly-scalable and minimal state core: Minimal state should
be kept inside the ISP's network. If the operator is rolling
out A+P incrementally, it is understood there may be state in
the core in the non-A+P part of such a roll-out.
4) Efficiency vs. complexity: Operators should have the
flexibility to trade off port multiplexing efficiency and
scalability and end-to-end transparency.
5) "Double-NAT" should be avoided: Multiple gateway devices
might be present in a path, and once one has done some
translation, those packets should not be re-translated.
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6) Legal traceability: ISPs must be able to provide the identity
of a customer from the knowledge of the IPv4 public address
and the port. This should have as low an impact as is
reasonable on storage by the ISP. We assume that NATs on
customer premises do not pose much of a problem, while
provider NATs need to keep additional logs.
7) IPv6 deployment should be encouraged. NAT444 strongly biases
the users to the deployment of RFC 1918 addressing.
Constraint 5 is important: while many techniques have been deployed
to allow applications to work through a NAT, traversing cascaded NATs
is crucial if NATs are being deployed in the core of a provider
network.
3.2. A+P Functions
The A+P architecture can be split into three distinct functions:
encaps/decaps, NAT, and signaling.
Encaps/decaps function: is used to forward port-restricted A+P-
packets over intermediate legacy devices. The encapsulation function
takes an IPv4 packet, looks up the IP and TCP/UDP headers, and puts
the packet into the appropriate tunnel. The state needed to perform
this action is comparable to a forwarding table. The decapsulation
device SHOULD check if the source address and port of packets coming
out of the tunnel are legitimate (e.g., see [BCP38]). Based on the
result of such a check, the packet MAY be forwarded untranslated, it
MAY be discarded or MAY be NATed. In this document we refer to a
device that provides this encaps/decaps functionality as Port-Range-
Router (PRR).
Network Address Translation (NAT) function: is used to connect legacy
end-hosts. Unless upgraded, end-hosts or end-systems are not aware
of A+P restrictions and therefore assume a full IP address. The NAT
function performs any address or port translation, including
Application Level Gateways (ALGs) whenever required. The state that
has to be kept to implement this function is the mapping for which
external addresses and ports have been mapped to which internal
addresses and ports, just as in CPEs embedding NAT today. A subtle,
but very important, difference should be noted here: the customer has
control over the NATing process or might choose to "bypass" the NAT.
If this is done, we call the NAT a large scale NAT (LSN). However,
if the NAT that does NOT allow the customer to control the
translation process, we refer to as a CGN.
Signaling function: is used in order to allow A+P-aware devices get
to know which ports are assigned to be passed through untranslated
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and what will happen to packets outside the assigned port-range
(e.g., could be NATed or discarded). Signaling may also be used to
learn the encapsulation method and any endpoint information needed.
In addition, the signaling function may be used to dynamically assign
the requested port-range.
3.3. Overview of the A+P Solution
As mentioned above, the core architectural elements of the A+P
solution are three separated and independent functions: the NAT
function, the encaps/decaps function, and the signaling function.
The NAT function is similar to a NAT as we know it today: it performs
a translation between two different address realms. When the
external realm is public IPv4 address space, we assume that the
translation is many-to-one, in order to multiplex many customers on a
single public IPv4 address. The only difference with a traditional
NAT (Figure 1) is that the translator might only be able to use a
restricted range of ports when mapping multiple internal addresses
onto an external one, e.g., the external address realm might be port-
restricted.
"internal-side" "external-side"
+-----+
internal | N | external
address <---| A |---> address
realm | T | realm
+-----+
Traditional NAT
Figure 1
The encaps/decaps function, on the other hand, is the ability to
establish a tunnel with another end-point providing the same
function. This implies some form of signaling to establish a tunnel.
Such signaling can be viewed as integrated with DHCP or as a separate
service. Section 3.3.1 discusses the constraints of this signaling
function. The tunnel can be an IPv6 or IPv4 encapsulation, a layer-2
tunnel, or some other form of softwire. Note that the presence of a
tunnel allows unmodified, naive, or even legacy devices between the
two endpoints.
Two or more devices which provide the encaps/decaps function and are
linked by tunnels to form an A+P subsystem. The function of each
gateway is to encapsulate and decapsulate respectively. Figure 2
depicts the simplest possible A+P subsystem, that is, two devices
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providing the encaps/decaps function.
+------------------------------------+
Private | +----------+ tunnel +----------+ | Public
address --|-| gateway |==========| gateway |-|-- address
realm | +----------+ +----------+ | realm
+------------------------------------+
A+P subsystem
A simple A+P subsystem
Figure 2
Within an A+P subsystem, the public address realm is extended by
using bits from the port number when forwarding packets. Each device
is assigned one address from the external realm and a range of port
numbers. Hence, devices which are part of an A+P subsystem can
communicate with the public realm without the need for address
translation (i.e., preserving end-to-end packet integrity): an A+P
packet originated from within the A+P subsystem can be simply
forwarded over tunnels up to the endpoint, where it gets decapsulated
and routed in the external realm.
3.3.1. Signaling
The following information needs to be available on all the gateways
in the A+P subsystem. It is expected that there will be a signaling
protocols such as [I-D.bajko-pripaddrassign],
[I-D.boucadair-dhcpv6-shared-address-option],
[I-D.boucadair-pppext-portrange-option], or
[I-D.wing-softwire-port-control-protocol].
The information that needs to be shared is the following:
o a set of public IPv4 addresses,
o for each IPv4 address a starting point for the allocated port-
range,
o number of delegated ports,
o optional key that enables partial or full preservation of entropy
in port randomization - see [I-D.bajko-pripaddrassign],
o lifetime for each IPv4 address and allocated port-set,
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o the tunneling technology to be used (e.g., "IPv6-encapsulation")
o addresses of the tunnel endpoints (e.g., IPv6 address of tunnel
endpoints)
o whether or not NAT function is provided by the gateway
o a device identification number and some authentication mechanisms
o a version number and some reserved bits for future use.
Note that the functions of encapsulation and decapsulation have been
separated from the NAT function. However, to accommodate legacy
hosts, NATing is likely to be provided at some point in the path;
therefore the availability or absence of NATing MUST be communicated
in signaling, as A+P is agnostic about NAT placement.
The port-ranges can be allocated in two different ways:
o If applications or end-hosts behind the CPE are not UPnPv2/NAT-PMP
aware, then the CPE SHOULD request ports via mechanisms, e.g. as
described in [I-D.bajko-pripaddrassign] and
[I-D.boucadair-pppext-portrange-option]. Note that different
port-ranges can have different lifetimes, and the CPE is not
entitled to use them after they expire - unless it refreshes those
ranges. It is up to the ISP to put mechanisms in place, that
determine what percentage of already allocated port-ranges should
be exhausted before a CPE may requests additional ranges, how
often the CPE can request additional ranges, and so on. (To
prevent Denial of Service attacks.)
o If applications behind the CPE are UPnPv2/NAT-PMP aware additional
ports MAY be requested through that mechanism. In this case the
CPE should forward those requests to the LSN and the LSN should
reply reporting if the requested ports are available or not (and
if they are not available some alternatives should be offered).
Here again, to prevent potential denial of service attacks,
mechanism should be in place to prevent UPnPv2/NAT-PMP packet
storms and fast port allocation. Detailed description of this
mechanism, called PCP is described in
[I-D.wing-softwire-port-control-protocol].
Whatever signaling mechanism is used inside the tunnels, DHCP, IPCP,
or PCP-based, synchronization between signaling server and PRR must
be established in both directions. For example, if we use DHCP as
signaling mechanism, the PRR must communicate to DHCP server at least
its IP range. The DHCP server then starts to allocate IPs and port-
ranges to CPEs and communicates back to the PRR which IP and port
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range have been allocated to which CPE, so the PRR knows to which
tunnel redirect incoming traffic. In addition, DHCP MUST also
communicate lifetimes of port-ranges assigned to CPE via the PRR.
DHCP server may be co-located with the PRR function to ease address
management and also to avoid the need to introduce a communication
protocol between the PRR and DHCP.
If UPnPv2/NAT-PMP is used as dynamic port allocation mechanism, the
PRR must also communicate to the DHCP (or IPCP) server to avoid those
ports. The PRR must somehow (DHCP or IPCP options) communicate back
to CPE that allocation of ports was successful, so CPE adds those
ports to existing port ranges.
Note that operation can be even simplified if a fixed length of port
ranges are assigned to all customers and no differentiation is
implemented based on port range length. In such case, the binding
table maintained by the PRR can be dynamically built upon the receipt
of a first packet form a port-restricted device.
3.3.2. Address Realm
Each gateway within the A+P subsystem manages a certain portion of
A+P address space, that is, a portion of IPv4 space which is extended
by borrowing bits from the port number. This address space may be a
single, port-restricted IPv4 address. The gateway MAY use its
managed A+P address space for several purposes:
o Allocation of a sub-portion of the A+P address space to other
authenticated A+P gateways in the A+P subsystem (referred to as
delegation). We call the allocated sub-portion delegated address
space.
o Exchange of (untranslated) packets with the external address
realm. For this to work, such packets MUST use source address and
port belonging to the non-delegated address space.
If the gateway is also capable of performing the NAT function, it MAY
translate packets arriving on an internal interface which are outside
of its managed A+P address space into non-delegated address space.
Hence, a provider may have 'islands' of A+P as they slowly deploy
over time. The provider does not have to replace CPE until they want
to provide the A+P function to an island of users or even to one
particular user in a sea of non-A+P users.
An A+P gateway ("A"), accepts incoming connections from other A+P
gateways ("B"). Upon connection establishment (provided appropriate
authentication), B would "ask" A for delegation of an A+P address.
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In turn, A will inform B about its public IPv4 address, and will
delegate a portion of its port-range to B. In addition, A will also
negotiate the encaps/decaps function with B (e.g., let B know the
address of the decaps device/other-end-point of the tunnel).
This could be implemented for example via a NAT-PMP or DHCP-like
solution. In general the following rule applies: A sub-portion of
the managed A+P address space is delegated as long as devices below
ask for it, otherwise private IPv4 is provided to support legacy
hosts.
In the following example, IPv4 address reserved for documentation
blocks defined in [RFC5737] are used.
private +-----+ +-----+ public
address ---| B |==========| A |--- Internet
realm +-----+ +-----+
Address space realm of A:
public IPv4 address = 192.0.2.1
port range = 0-65535
Address space realm of B:
public IPv4 address = 192.0.2.1
port range = 2560-3071
Configuration example
Figure 3
Figure 3 illustrates a sample configuration. Note that A might
actually consist of three different devices: one that handles
signaling requests from B; one device that performs encapsulation and
decapsulation; and, if provided, one device that performs NATing
function (e.g., LSN). Packet forwarding is assumed to be as follows:
In the "out-bound" case, a packet arrives from the private address
realm to B. As stated above, B has two options: it can either apply
or not apply the NAT function. The decision depends upon the
specific configuration and/or the capabilities of A and B. Note that
NAT functionality is required to support legacy hosts, however, this
can be done at either of the two devices A or B. The term NAT refers
to translating the packet into the managed A+P address (B has address
192.0.2.1 and ports 2560-3071 in the example above). We then have
two options:
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1) B NATs the packet. The translated packet is then tunneled to A.
A recognizes that the packet has already been translated, because
the source address and port match the delegated space. A
decapsulates the packet and releases it in the public Internet.
2) B does not NAT the packet. The untranslated packet is then
tunneled to A. A recognizes that the packet has not been
translated, so A forwards the packet to a co-located NATing
device, which translates the packet and routes it in the public
Internet. This device, e.g. - an LSN, has to store the mapping
between the source port used to NAT and the tunnel where the
packet came from, in order to correctly route the reply. Note
that A cannot use a port number from the range that has been
delegated to B. As a consequence A has to assign a part of its
non-delegated address space to the NATing function.
"Inbound" packets are handled in the following way: a packet from the
public realm arrives at A. A analyzes the destination port number to
understand whether the packet needs to be NATed or not.
1) If the destination port number belongs to the range that A
delegated to B, then A tunnels the packet to B. B NATs the packet
using its stored mapping and forwards the translated packet to
the private domain.
2) If the destination port number is from the address space of the
LSN, then A passes the packet on to the co-located LSN which uses
its stored mapping to NAT the packet into the private address
realm of B. The appropriate tunnel is stored as well in the
mapping of the initial NAT. The LSN then encapsulates the packet
to B, which decapsulates it and normally routes it within its
private realm.
3) Finally, if the destination port number neither falls in a
delegated range, nor into the address range of the LSN, A
discards the packet. If the packet is passed to the LSN, but no
mapping can be found, the LSN discards the packet.
Observe that A must be able to receive all IPv4 packets destined to
the public IPv4 address (192.0.2.1 in the example), so that it can
make routing decisions according to the port number. On the other
hand, B receives IPv4 packets destined to the public IPv4 address
only via the established tunnel with A. In other words, B uses the
public IPv4 address just for translation purposes, but it is not used
to make routing decisions. This allows us to keep the routing logic
at B as simple as described above, while enabling seamless
communication between A+P devices sharing the same public IPv4
address.
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private +-----+ +-----+ public
address ---| B |==========| A |--- Internet
realm 1 +-----+ +-----+
|
private +-----+ |
address ---| C |============/
realm 2 +-----+
Address space realm of A:
public IPv4 address = 192.0.2.1
port range = 0-65535
Address space realm of B:
public IPv4 address = 192.0.2.1
port range = 2560-3071
Address space realm of C:
public IPv4 address = 192.0.2.1
port range = 0-2559
Hierarchical A+P
Figure 4
Consider the example shown in Figure 4. Here both B and C use the
encaps/decaps function to establish a tunnel with A, and they are
assigned the same public IPv4 address with different, non-overlapping
port-ranges. Assume that a host in B's private realm sends a packet
destined to address 192.0.2.1 and port 2000, and that B has been
instructed to NAT all packets destined to 192.0.2.1. Under these
assumptions, B receives the packet and NATs it using its own public
IPv4 address (192.0.2.1) and a port selected from its configured
port-range (e.g., 3000). B then tunnels the translated packet to A.
When A receives the packet via the tunnel, it looks at the
destination address and port, recognizes C's delegated range, and
then tunnels the packet to C. Observe that, apart from stripping the
tunnel header, A handles the packet as if it came from the public
Internet. When C receives the packet, it NATs the destination
address into one address chosen from its private address realm, while
keeping the source address (192.0.2.1) and port (3000) untranslated.
Return traffic is handled the same way. Such a mechanism allows
hosts behind A+P devices to communicate seamlessly even when they
share the same public IPv4 address.
Please refer to Section 4 for a discussion of an alternative A+P
mechanism that does not incur in path stretches penalties for intra-
domain communication.
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Internet-Draft A+P Addressing Extension January 20113.3.3. Reasons for Allowing Multiple A+P Gateways
Since each device in an A+P subsystem provides the encaps/decaps
function, new devices can establish tunnels and become in turn part
of an A+P subsystem. As noted above, being part of an A+P subsystem
implies the capability of talking to the external address realm
without any translation. In particular, as described in the previous
section, a device X in an A+P subsystem can be reached from the
external domain by simply using the public IPv4 address and a port
which has been delegated to X. Figure 5 shows an example where three
devices are connected in a chain. In other words, A+P signaling can
be used to extend end-to-end connectivity to the devices which are in
an A+P subsystem. This allows A+P-aware applications (or OSes)
running on end hosts to enter an A+P subsystem and exploit
untranslated connectivity.
There are two modes for end-hosts to gain fine-grained control of
end-to-end connectivity. The first is where actual end-hosts perform
the NAT function and the encaps/decaps function which is required to
join the A+P subsystem. This option works in a similar way to the
NAT-in-the-host trick employed by virtualization software such as
VMware, where the guest operating system is connected via a NAT to
the host operating system. The second mode is applications which
autonomously ask for an A+P address and use it to join the A+P
subsystem. This capability is necessary for some applications that
require end-to-end connectivity (e.g., applications that need to be
contacted from outside).
+---------+ +---------+ +---------+
internal | gateway | | gateway | | gateway | external
realm --| 1 |======| 2 |======| 3 |-- realm
+---------+ +---------+ +---------+
An A+P subsystem with multiple devices
Figure 5
Whatever the reasons might be, the Internet was built on a paradigm
that end-to-end connectivity is important. A+P makes this still
possible in a time where address shortage forces ISPs to use NATs at
various levels. In such sense, A+P can be regarded as a way to
bypass NATs.
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+---+ (customer2)
|A+P|-. +---+
+---+ \ NAT|A+P|-.
\ +---+ |
\ | forward if in-range
+---+ \+---+ +---+ /
|A+P|------|A+P|----|A+P|----
+---+ /+---+ +---+ \
/ NAT if necessary
/ (cust1) (prov. (e.g., provider NAT)
+---+ / router)
|A+P|-'
+---+
A complex A+P subsystem
Figure 6
Figure 6 depicts a complex scenario, where the A+P subsystem is
composed by multiple devices organized in a hierarchy. Each A+P
gateway decapsulates the packet and then re-encapsulates it again to
the next tunnel.
A packet can either be NATed when it enters the A+P subsystem, or at
intermediate devices, or when it exits the A+P subsystem. This could
be for example a gateway installed within the provider's network,
together with a LSN. Then each customer operates its own CPE.
However, behind the CPE applications might also be A+P-aware and run
their own A+P-gateways, which enables them to have end-to-end
connectivity.
One limitation applies, if "delayed translation" is used (e.g.,
translation at the LSN instead of the CPE). If devices using
"delayed translation" want to talk to each other they SHOULD use A+P
addresses or out-of-band addressing.
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Internet-Draft A+P Addressing Extension January 20113.3.4. Overall A+P Architecture
A+P architecture
IPv4 Full-A+P AFTR CGN
| | | |
<-- Full IPv4 ---- Port range ---- Port range ---- Provider --->
allocated & dynamic & LSN NAT ONLY
allocation (NAT on CPE (No mechanism)
(no NAT) (NAT on CPE) and on LSN) for customer to
bypass CGN)
Figure 7: A+P overall architecture
The A+P architecture defines various deployment options within an
ISP. Figure 7 shows the spectrum of deployment options. On the far
left is the common deployment method for broadband subscribers today,
an IPv4 address unrestricted with full port-range. Full-A+P refers
to a port-range allocation from the ISP. The customer must operate
an A+P-aware CPE device and no NATing functionality is provided by
the ISP. AFTR, such as DS-Lite [I-D.ietf-softwire-dual-stack-lite],
is a hybrid. There is NAT present in the core (in this document
referred to as LSN), but the user has the option to "bypass" that NAT
in one form or an other, for example via A+P, NAT-PMP, etc...
Finally, a service provider which only deploys CGN, will place a NAT
in the providers core and does not allow the customer to "bypass" the
translation process or modify ALGs on the NAT. The customer is
provider-locked. Notice that all options (besides full IPv4) require
some form of tunneling mechanism (e.g., 4in6) and a signaling
mechanism (see Section 3.3.1).
4. Stateless A+P Mapping Function4.1. Stateless A+P Mapping gateway (SMAP) Function description
SMAP stands for Stateless A+P Mapping. This function is responsible
to encapsulate (Resp., decapsulate), in a stateless scheme, IPv4
packets in (Resp. from) IPv6 ones. A SMAP function may be hosted in
a PRR, end-user device, etc.
Stateless A+P Mapping gateway (SMAP) consists in two basic functions
as described in Figure 8.
1. SMAP encapsulates an IPv4 packet, destined to a shared IPv4
address, in IPv6 one. The IPv6 source address is constructed using
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an IPv4-Embedded IPv6 address [I-D.ietf-behave-address-format] from
the IPv4 source address and port number plus the IPv6 prefix which
has been provisioned to the node performing the SMAP function. The
destination IPv6 address is constructed using the shared IPv4
destination address and port number plus the IPv6 prefix which has
been provisioned to the SMAP function and which is dedicated to IPv4
destination addresses.
2. SMAP extracts IPv4 incoming packets from IPv6 incoming ones which
have IPv6 source addresses belonging to the prefix of the node
performing the SMAP function. Extracted IPv4 packets are then
forwarded to the point identified by the IPv4 destination address and
port number.
+-------------------+
| |----IPv6---\
----IPv4---\| |----IPv4---\\
-----------/| |-----------//
| |-----------/
| SMAP |
| | /--IPv6-----
/---IPv4----| |//---IPv4----
\-----------| |\\-----------
| | \-----------
+-------------------+
Figure 8: Stateless A+P Mapping Gateway Function
A SMAP-enabled node will perform the stateless 6/4 mapping function
for all public shared IPv4 addresses for which it was designated as a
stateless 6/4 mapping gateway.
To perform stateless 6/4 mapping function a SMAP gateway must:
o be provided with an IPv6 prefix (i.e., Pref6). The SMAP gateway
uses this prefix to construct IPv6 source addresses for all IPv4
shared addresses for which it was designated as a SMAP gateway. The
IPv6 prefix may be provisioned statically or dynamically (e.g., DHCP)
o be able to know the IPv6 prefix of the node serving as another SMAP
gateway for IPv4 destination addresses. This prefix may be known in
various ways:
* Default or Well Known Prefix (i.e., 64:ff9b::/96) which was
provisioned statically or dynamically;
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* Retained at the reception of incoming IPv4-in-IPv6 encapsulated
packets;
* Discovered at the communication starting thanks to mechanisms as
DNS resolution for example.
When the SMAP-enabled node receives IPv4 packets with IPv4 source
addresses for which it was not designated as a SMAP gateway, it will
not perform stateless 6/4 mapping function for those packets. Those
packets will be handled in a classical way (i.e., forwarded, dropped
or locally processed).
When the SMAP-enabled node receives IPv6 packets with IPv6 addresses
which do not match with its IPv6 prefix, it will not perform the
stateless 6/4 mapping function for those packets. Those packets will
be handled in a classical way (i.e., forwarded, dropped or locally
processed).
4.2. Implementation Mode
In this configuration, the node A performs the stateless mapping
function on the received IPv4 traffic (encapsulated in IPv6 packets)
before forwarding to the node B. The node B performs the stateless
mapping function on the received IPv6 traffics (extracting IPv4
packets) before forwarding the IPv4 traffic to the destination
identified by the IPv4 destination address and port number. In the
opposite direction and as previously, the node B performs the
stateless mapping function on the received IPv4 traffics
(encapsulating in IPv6 packets) before forwarding to the node A. The
node A performs the stateless mapping function on the received IPv6
traffic (extracting IPv4 packets) before forwarding the IPv4 traffic
to the point identified by the IPv4 destination address and port
number. In this case, only IPv6 traffic is managed in the network
segment between the nodes A and B.
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+------+ +------+
| |----IPv6---\ | |
----IPv4---\| |----IPv4---\\| |----IPv4---\
-----------/| |-----------//| |-----------/
| |-----------/ | |
| SMAP | | SMAP |
| | /----IPv6---| |
/---IPv4----| |//---IPv4----| |/---IPv4----
\-----------| |\\-----------| |\-----------
| | \-----------| |
+------+ +------+
node A node B
Figure 9
Several deployment scenarios of the SMAP function may be envisaged in
the context of Port Range based solutions:
o A SMAP function is embedded in a port-restricted device. Other
SMAP-enabled nodes are deployed in the boundaries between IPv6-
enabled realms and IPv4 ones. This scenario may be particularly
deployed for intra-domain communications so as to interconnect
heterogeneous realms (i.e., IPv6/IPv4) within the same AS.
o A SMAP function is embedded in a port-restricted device. Other
SMAP-enabled nodes are deployed in the interconnection segment (with
adjacent IPv4-only ones) of a given AS. This deployment scenario is
more suitable for service providers targeting the deployment of IPv6
since it eases the migration to full IPv6. Core nodes are not
required to activate anymore both IPv4 and IPv6 transfer
capabilities.
Other considerations regarding the interconnection of SMAP-enabled
domains should be elaborated. The following provides a non
exhaustive list of interconnection schemes.
o The interconnection of two domains implementing the SMAP function
may be deployed via IPv4 Internet (Figure 10): This means that IPv4
packets encapsulated in IPv6 one are transferred using IPv6 until
reaching the first SMAP-node. Then these packets are de-
encapsulated and are forwarded using IPv4 transfer capabilities. A
remote SMAP-enabled node will receive those packets and proceeds to
an IPv4-in-IPv6 encapsulation. These packets are then routed
normally until reaching the port-restricted devices which de-
encapsulates the packets.
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Internet-Draft A+P Addressing Extension January 20114.3. Towards IPv6-only Networks
The deployment of SMAP function allows for smooth migration of
networks to IPv6-only scheme while maintaining the delivery of IPv4
connectivity services to customers. The delivery of IPv4
connectivity services over an IPv6-only network does not require any
stateful function to be deployed on the core network. Owing to this
A+P mode, both the IPv4 service continuity and migration to an IPv6-
only deployment model are facilitated.
4.4. PRR: On Stateless and Binding Table Modes
o Stateless Mode
Complete stateless mapping implies that the IPv4 address and the
significant bits coding the port range are reflected inside the IPv6
prefix assigned to the port-restricted device. Two alternatives are
offered when such a stateless mapping is to be enabled:
- either using the IPv6 prefix already used for native IPv6 traffic,
- or provide two prefixes to the port-restricted device: one for the
native IPv6 traffic and one for the IPv4 traffic.
Note that:
- Providing two IPv6 prefixes has the advantages of allowing a /64
prefix for the port-restricted device along with another prefix
(e.g., a /56 or /64) for native IPv6 traffic. This alternative
spares the service provider to relate the native IPv6 traffic
addressing plan to the IPv4 addressing plan. The drawback is the
burden to allocate two prefixes to each port-restricted device and to
route them. In addition, an address selection issue may be
encountered.
- Providing one prefix for both needs (e.g., a /56 or a /64) spares
the service provider to handle two types of IPv6 prefix for the port-
restricted device and in routing tables. But the drawback is that it
somewhat links strongly the IPv4 addressing plan to the allocated
IPv6 prefixes.
o Binding Table Mode
Another alternative is to assign a "normal" IPv6 prefix to the port-
restricted device and to use a binding table, which can be hosted by
a service node, to correlate the (shared IPv4 address, Port Range)
with an IPv6 address part of the assigned IPv6 prefix. For
scalability reasons, this table should be instantiated within PRR-
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enabled nodes which are close to the port-restricted devices. The
number of required entries if hosted at interconnection segment would
be equal to the amount of subscribed users (one per port-restricted
device).
4.5. General recommendations on SMAP
If Stateless A+P Mapping (SMAP) type of implementation is deployed
over intermediate IPv6-ONLY-capable devices, it is recommended that
default-routes are configured and IPv4 routing table is not "leaked"
into IPv6 routing table in terms to have reachability for the packets
going towards the internet.
5. Deployment Scenarios5.1. A+P Deployment Models5.1.1. A+P for Broadband Providers
Some large broadband providers will not have enough public IPv4
address space to provide every customer with a single IP. The
natural solution is sharing a single IP address among many customers.
Multiplexing customers is usually accomplished by allocating
different port numbers to different customers somewhere within the
network of the provider.
It is expected that, when the provider wishes to enable A+P for a
customer or a range of customers, the CPE can be upgraded or replaced
to support A+P encaps/decaps functionality. Ideally the CPE also
provides NATing functionality. Further, it is expected that at least
another component in the ISP network provides the corresponding A+P
functionality, and hence is able to establish an A+P subsystem with
the CPE. This device is referred to as A+P router or port-range
router (PRR), and could be located close to PE routers. The core of
the network MUST support the tunneling protocol (which SHOULD be
IPv6, as per Constraint 7) but MAY be another tunneling technology
when necessary. In addition, we do not wish to restrict any
initiative of customers who might want to run an A+P-capable network
on or behind their CPE. To satisfy both Constraints 1 and 2
unmodified legacy hosts should keep working seamlessly, while
upgraded/new end-systems should be given the opportunity to exploit
enhanced features.
5.1.2. A+P for Mobile Providers
In the case of mobile service provider the situation is slightly
different. The A+P border is assumed to be the gateway (e.g., GGSN/
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PDN GW of 3GPP, or ASN GW of WiMAX). The need to extend the address
is not within the provider network, but on the edge between the
mobile phone devices and the gateway. While desirable, IPv6
connectivity may or may not be provided.
For mobile providers we use the following terms and assumptions:
1. Provider Network (PN)
2. Gateway (GW)
3. Mobile Phone device (phone)
4. Devices behind phone, e.g., laptop computer connecting via phone
to Internet.
We expect that the gateway has a pool of IPv4 addresses and is always
in the data-path of the packets. Transport between the gateway and
phone devices is assumed to be an end-to-end layer-2 tunnel. We
assume that phone as well as gateway can be upgraded to support A+P.
However, some applications running on the phone or devices behind the
phone (such as laptop computers connecting via the phone), are not
expected to be upgraded. Again, while we do not expect that devices
behind the phone will be A+P aware/upgraded we also do not want to
hinder their evolution. In this sense the mobile phone would be
comparable to the CPE in the broadband provider case; the gateway to
the PRR/LSN box in the network of the broadband provider.
5.1.3. A+P from the Provider Network Perspective
ISPs suffering from IPv4 address space exhaustion are interested in
achieving a high address space compression ratio. In this respect,
an A+P subsystem allows much more flexibility than traditional NATs:
the NAT can be placed at the customer, and/or in the provider
network. In addition hosts or applications can request ports and
thus have untranslated end-to-end connectivity.
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+---------------------------+
private | +------+ A+P-in +-----+ | dual-stacked
(RFC1918) --|-| CPE |==-IPv6-==| PRR |-|-- network
space | +------+ tunnel +-----+ | (public addresses)
| ^ +-----+ |
| | IPv6-only | LSN | |
| | network +-----+ |
+----+----------------- ^ --+
| |
on customer within provider
premises and control network
A simple A+P subsystem example
Figure 12
Consider the deployment scenario in Figure 12, where an A+P subsystem
is formed by the CPE and a PRR within the ISP core network,
preferably close to the customer edge, and represents the border from
where on packets are forwarded based on address and port. The
provider MAY deploy a LSN co-located with the PRR to handle packets
that have not been translated by the CPE. In such a configuration,
the ISP allows the customer to freely decide whether the translation
is done at the CPE or at the LSN. In order to establish the A+P
subsystem, the CPE will be configured automatically (e.g. via a
signaling protocol, that conforms to the requirements stated above).
Note that the CPE in the example above is only provisioned with an
IPv6 address on the external interface.
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+------------ IPv6-only transport ------------+
| +---------------+ | | |
| |A+P-application| | +--------+ | +-----+ | dual-stacked
| | on end-host |=|==| CPE w/ |==|==| PRR |-|-- network
| +---------------+ | +--------+ | +-----+ | (public addresses)
+---------------+ | +--------+ | +-----+ |
private IPv4 <-*--+->| NAT | | | LSN | |
address space \ | +--------+ | +-----+ |
for legacy +|--------------|----------+
hosts | |
| |
end-host with | CPE device | provider
upgraded | on customer | network
application | premises |
An extended A+P subsystem with end-host running A+P-aware
applications
Figure 13
Figure 13 shows an example of how an upgraded application running on
a legacy end-host can connect to another host in the public realm.
The legacy host is provisioned with a private IPv4 address allocated
by the CPE. Any packet sent from the legacy host will be NATed
either at the CPE (if configured to do so), or at the LSN (if
available).
An A+P-aware application running on the end-host MAY use the
signaling described in Section 3.3.1 to connect to the A+P-subsystem.
In this case, the application will be delegated some space in the A+P
address realm, and will be able to contact the public realm (i.e.,
the public Internet) without the need for translation.
Note that part of A+P signaling is that the NATs are optional.
However, if neither the CPE nor the PRR provides NATing
functionality, then it will not be possible to connect legacy end-
hosts.
To enable packet forwarding with A+P, the ISP MUST install at its A+P
border a PRR which encaps/decaps packets. However, to achieve a
higher address space compression ratio and/or to support CPEs without
NATing functionality, the ISP MAY decide to provide an LSN as well.
If no LSN is installed in some part of the ISP's topology, all CPE in
that part of the topology MUST support NAT functionality. For
reasons of scalability, it is assumed that the PRR is located within
the access-portion of the network. The CPE would be configured
automatically (e.g. via an extended DHCP or NAT-PMP, which has the
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signaling requirements stated above) with the address of the PRR, and
if a LSN is being provided or not. Figure 12 illustrates a possible
deployment scenario.
5.2. Dynamic Allocation of Port Ranges
Allocating a fixed number of ports to all CPEs may lead to exhaustion
of ports for high usage customers. This is a perfect recipe for
upsetting more demanding customers. On the other hand, allocating to
all customers ports sufficient to match the needs of peak users will
not be very efficient. A mechanism for dynamic allocation of port
ranges allows the ISP to achieve two goals; a more efficient
compression ratio of number of customers on one IPv4 address and, on
the other hand, not limiting the more demanding customers'
communication.
Additional allocation of ports, or port ranges may be made after an
initial static allocation of ports.
The mechanism would prefer allocations of port ranges from the same
IP address as the initial allocation. If it is not possible to
allocate an additional port range from the same IP, then mechanism
can allocate a port range from another IP within the same subnet.
With every additional port range allocation, the PRR updates its
routing table. The mechanism for allocating additional port ranges
may be part of normal signaling that is used to authenticate CPE to
ISP.
The ISP controls the dynamic allocation of port ranges by the PRR by
setting the initial allocation size and maximum number of allocations
per CPE, or the maximum allocations per subscription, depending on
subscription level. There is a general observation that the more
demanding customer uses around 1024 ports when heavily communicating.
So, for example, a first suggestion might be 128 ports initially and
then dynamic allocations of ranges of 128 ports up to 511 more
allocations maximum. A configured maximum number of allocations
could be used to prevent one customer acting in destructive manner
should they become infected. The maximum number of allocations might
also be more finely grained, with parameters of how many allocations
a user may request per some time frame. If this is used, evasive
applications may need to be limited in their bad behavior, for
example one additional allocation per minute would considerably slow
a port request storm.
There is likely no minimum request size. This is because A+P-aware
applications running on end-hosts MAY request a single port (or a few
ports) for the CPE to be contacted on (e.g., VoIP clients register a
public IP and a single delegated port from the CPE, and accept
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incoming calls on that port). The implementation on the CPE or PRR
will dictate how to handle such requests for smaller blocks: For
example, half of available blocks might be used for "block-
allocations", 1/6 for single port requests, and the rest for NATing.
Another possible mechanism to allocate additional ports is UPnP/
NAT-PMP (as defined in Section 3.3.1), if applications behind CPE
support it. In case of the LSN implementation (DS-Lite), as
described in the A+P overall architecture section, signaling packets
are simply forwarded by the CPE to the LSN and back to the host
running the application which requested the ports, and PRR allocates
requested port to appropriate CPE. The same behavior may be chosen
with AFTR, if requested ports are outside of static initial port
allocation. If a full A+P implementation is selected, than UPnPv2/
NAT-PMP packets are accepted by the CPE, processed, and the requested
port number is communicated through normal signaling mechanism
between CPE and PRR tunnel endpoints (PCP).
5.3. Example of A+P-forwarded Packets
This section provides a detailed example of A+P setup, configuration,
and packet flow from an end-host connected to A+P Service Provider to
any host in the IPv4 Internet, and how the return packets flow back.
The following example discusses an A+P-unaware end-host, where the
NATing is done at the CPE. Figure 14 illustrates how the CPE
receives an IPv4 packet from the end-user device. We first describe
the case where the CPE has been configured to provide the NAT
functionality (e.g., by the customer through interaction with a
website, or automatic signaling). In the following, we call a packet
which is translated at the CPE an A+P-forwarded packet, an analogy
with the port-forwarding function employed in today's CPEs. Upon
receiving a packet from the internal interface, the CPE translates,
encapsulates and forwards it to the PRR. The NAT on the CPE is
assumed to have a default route to the public realm through its
tunnel interface.
When the PRR receives the A+P-forwarded packet, it de-capsulates the
inner IPv4 packet and checks the source address. If the source
address does match the range assigned to A+P enabled CPEs, then the
PRR simply forwards the decapsulated packet onward. This is always
the case for A+P-forwarded packets. Otherwise, the PRR assumes that
the packet is not A+P-forwarded, and passes it to the LSN function,
which in-turn translates and forward the packet based on the
destination address. Figure 14 shows the packet flow for an outgoing
A+P-forwarded packet.
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ECHO and ECHO reply are more problematic. For ECHO, the A+P gateway
device must rewrite the "Identifier" and perhaps "Sequence Number"
fields in the ICMP request, treating them as if they were port
numbers. This way, the PRR can build the correct A+P address for the
returning ECHO replies, so they can be correctly routed back to the
appropriate host in the same way as TCP/UDP packets. Pings
originated from the Public Realm (Internet) towards an A+P device are
not supported.
5.3.3. Fragmentation
In order to deliver a fragmented IP packet to its final destination
(among those having the same IP address), the PRR should activate a
dedicated procedure similar to the one used by
[I-D.ietf-behave-v6v4-xlate-stateful], section 3.5 in a sense that it
should reassemble the fragments in order to look at the destination
port number.
Note that it is recommended to use a PMTUD path discovery mechanism
(e.g., [RFC1191]).
Security issues related to fragmentation are out of scope of this
document. For more details, refer to [RFC1858].
5.3.4. Limitations of the A+P approach
One limitation that A+P shares with any other IP address-sharing
mechanism is the availability of well-known ports. In fact, services
run by customers that share the same IP address will be distinguished
by the port number. As a consequence, it will be impossible for two
customers who share the same IP address to run services on the same
port (e.g., port 80). Unfortunately, working around this limitation
usually implies application-specific hacks (e.g., HTTP and HTTPS
redirection), discussion of which is out of the scope of this
document. Of course, a provider might charge more for giving a
customer the well-known port range, 0..1024, thus allowing the
customer to provide externally available services. Many applications
require the availability of well known ports. However, those
applications are not expected to work in A+P environment unless they
can adapt to work with different ports. However, such application do
not work behind today's NATs either.
Another problem which is common to all NATs is coexistence with
IPsec. In fact, a NAT which also translates port numbers prevents AH
and ESP from functioning properly, both in tunnel and in transport
mode. In this respect, we stress that, since an A+P subsystem
exhibits the same external behavior as a NAT, well-known workarounds
(such as [RFC3715]) can be employed.
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Internet-Draft A+P Addressing Extension January 20116. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
7. Security Considerations
With CGN/LSNs, tracing hackers, spammers and other criminals will be
difficult, requiring logging, recording, and storing of all
connection based mapping information. The need for storage implies a
tradeoff. On one hand, the LSNs can manage addresses and ports as
dynamically as possible in order to maximize aggregation. On the
other hand, the more quickly the mapping between private and public
space changes, the more information needs to be recorded. This would
not only cause concern for law enforcement services, but also for
privacy advocates.
A+P offers a better set of tradeoffs. All that needs to be logged is
the allocation of a range of port numbers to a customer. By design,
this will be done rarely, improving scalability. If the NAT
functionality is moved further up the tree, the logging requirement
will be as well, increasing the load on one node, but giving it more
resources to allocate to a busy customer, perhaps decreasing the
frequency of allocation requests.
The other extreme is A+P NAT on the customer premises. Such a node
would be no different than today's NAT boxes, which do no such
logging. We thus conclude that A+P is no worse than today's
situation, while being considerably better than CGNs.
8. Authors
This document has 9 primary authors, which is not allowed in the
header of Internet-Drafts. This is the list of actual authors of
this document.
Gabor Bajko
Nokia
Email: gabor(dot)bajko(at)nokia(dot)com
Mohamed Boucadair
France Telecom
3, Av Francois Chateaux
Rennes 35000
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